Study: Snow depth in Sierra Nevada has been relatively steady past 100 years

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LAKE TAHOE — It's well known that California, and especially the Sierra Nevada, can have wild fluctuations in weather. But, according to one new study, the average snowfall depth has been relatively steady over the past 130 years.

“There isn't a trend significantly different from zero for the whole period,” said the study's author, John Christy, director of the Earth System Science Center at the University of Alabama. “I also looked at just the past 50 years and there is no trend over this recent stretch either.”

Christy, a critic of global warming science, contends the study's findings contradict concerns that snowfall in the Sierra Nevada is disappearing due to manmade climate change.

A summary of climate change in California's 2009 Water Plan Update acknowledges uncertainties about how increased greenhouse gas concentrations may affect precipitation, but finds “there is a very high confidence that higher temperatures will lead to dramatic changes in the snowfall and snowmelt dynamics in watersheds with substantial snow.”

“Higher temperatures will have several major effects: they will increase the ratio of rain to snow, delay the onset of the snow season, accelerate the rate of spring snowmelt, and shorten the overall snowfall season, leading to more rapid and earlier seasonal runoff,” according to the summary.

Christy gathered his data set from snow depth records kept by stations along the Southern Pacific Railroad.

“They took great care to measure snowfall because they had to know how much snow fell before sending trains through the mountain passes,” Christy said. “No one else had looked at this data in detail. The records are pretty thorough and the measuring tools — a device resembling a tall, sturdy yardstick — are easy to use and obviously don't need power, so there aren't many gaps in the record.”

He also used data from hydro-power and regional water systems, logging and mining companies and the National Weather Service stations and volunteers. Christy had to manually enter much of the hand-written data into a computer.

To analyze the information, Christy divided the state into 18 regions based on the snowfall and the quality of records for that region. Six or seven of the regions had good, robust data with at least five to 15 stations with records, he said.

Looking at both the 130-year record and the most recent 50-year record — which includes the 1975 to 2000 period when global temperatures rose — the California data show no long-term changes in snowfall in any region, according to a statement released about the study.

“California has huge year-to-year variations and that's expected to continue,” Christy said. “California is having a snow drought so far this winter, while last year the state had much heavier than normal snowfall. But over the long term, there just isn't a trend up or down.

The accuracy of the study has been questioned by California researchers who study the state's snow.

“John Christy failed to find a downward trend in California snowfall because he is looking in the wrong places,” said snow researcher and occasional Tahoe Environmental Research Center consultant Bob Coats in a letter. “As climate scientist Michael Dettinger pointed out, snowfall depth is a poor variable.”

Coats argued that trends measured in inches of water falling as snow would be a more accurate way to measure changes in the Sierra Nevada. He found a strong downward trend in precipitation as snow in a study he did with data from 1910 to 2007, he said. The probably that this trend is due to chance alone is extremely slim, he added.

Christy's study was published in the American Meteorological Society's Journal of Hydrometeorology.